Introduction Much attention has been focused on the contr ibution of sensory neurons and their synaptic connections to defensive reflexes in Aplysia. The two best understood withdrawal reflexes in terms of their underlying circuitry and their roles in learning and memory, are the siphon-elicited siphon-gill withdrawal reflex and the tail-elicited tail-siphon withdrawal reflex (Fig. 1; for review, see Kandel and Schwartz 1982; Carew and Sahley 1986; Hawkins et al. 1986; Byrne 1987; Byrne et al. 1991; Frost and Kandel 1995; Byrne and Kandel 1996). Both reflex responses have been used extensively to analyze the neural and molecular mechanisms of sensitization, a simple form of nonassociative learning in which an unexpected (often noxious) stimulus enhances the response to a subsequent test stimulus (for review, see Byrne et al. 1993; Hawkins et al. 1993). Another form of nonassociative learning, habituation, has also been the subject of study, although to a lesser extent. The duration of both forms of learning can be brief (minutes) or long (days), depending on the training protocol. Much of the work on the mechanisms underlying sensitization in these two reflexes has focused on training-related modifications of the central sensory neurons. In particular, sensitizing stimuli act to enhance both sensory neuron excitability and transmitter release (for review, see Byrne et al. 1993; Hawkins et al. 1993). These changes occur via the activation of second messenger systems in a timeand state-dependent manner (Baxter and Byrne 1990; Braha et al. 1990; Ghirardi et al. 1992; Pieroni and Byrne 1992; Sugita et al. 1992, 1994; Byrne and Kandel 1996). Several of these storage processes operate in parallel to maintain both the shortand long-term components of the memory for sensitization (Frost et al. 1985; Greenberg et al. 1987; Schwartz and Greenberg 1987; Scholz and Byrne 1988; Bailey et al. 1992; Kennedy et al. 1992; Byrne et al. 1993; Alberini et al. 1994; O'Leary et al. 1995). More recent studies have begun to explore the role of circuit interneurons in both the mediation and modulation of these withdrawal responses. This review will address several questions about the roles played by the interneurons in these circuits. What do the interneurons contr ibute to the mediation of these reflexes? Does their contr ibution differ qualitatively from that made by the monosynaptic sensory neuron to motor neuron connections? Do the interneurons themselves serve as sites of memory storage and, if so, do they use the same storage mechanisms as those utilized by the sensory neurons? What role do interneurons play in learning-related modulation of the circuits? The findings reviewed here indicate that interneurons make important and distinctive contributions to the mediation of withdrawal reflexes in Aplysia. Furthermore, some of the interneurons serve as sites of information storage in habituation and sensitization, storing different 1Corresponding author, information, and utilizing different types of plasticity than the sensory